A smooth and circular conduit often has a tendency toward self-burial, but the effect often is insufficient, so that the conduit is easily exposed again and/or the time for self-burying is very long, with the inherent danger of damage by anchors, fishing gears or the like. In view regulations of several governments that require that a conduit be buried so much within one year that there is at least a covering layer of a height of 20 cm above it, various methods of burying conduits have been proposed.
Prior methods of burial have included the making of trenches by dredging, by cutting with rotary cutters or by strong water jets and to position the pipeline or conduit therein. Such prior methods have been realized in such a way that the conduit has lowered to the bottom underwater during the preparation of such a trench. After positioning of the conduit in the trench, the trench may be filled up above the conduit with the material which first has been removed for making the trench so as to obtain sufficient earth material above the conduit, so that it is not again exposed by the stirring action of the water flows along the bottom. These traditional methods of burying conduits suffered from numerous problems, including, but not limited to, adverse environmental impact and cost.
A method of self-burial of a conduit in a seabed using protruding parts such as fins was disclosed in U.S. Pat. No. 4,648,745, entitled “Method and Device for Burying a Conduit in the Bottom of a Waterbed,” which is incorporated by reference herein in its entirety. That patent disclosed a method in which protrusions such as fins are provided on or in the vicinity of the conduit, which cause disturbances in the flow of water near the conduit to erode the bottom of the waterbed in such a way that the conduit will be buried.
It has appeared that by application of the invention horizontal water flows near the conduit generate flow disturbances and/or turbulences, swirling up, dragging with it and removing the bottom material to the side of and in many cases also below the conduit, so that the conduit sinks into the bottom by its own weight, which process proceeds until the conduit is sufficiently buried, after which the depression, which is thereby formed in the proximity of the conduit, silts up so that so much earth will cover the conduit that it remains buried safely.
The method of that patent was realized in such a way that the protruding parts were provided at a horizontal distance from the conduit and extending substantially parallel thereto, provided on or in the bottom, and this possibility could be combined with the application of one or more protruding parts on the conduit itself. That design was believed to be particularly advantageous when the horizontal flows along the bottom in the water are in one direction only instead of switching to and fro by tide currents.
The patent further noted that tilting of the conduit may occur in certain environments, and if the conduit may easily tilt to a more considerable extent, it was possible to provide a number of protruding parts, distributed over a greater part of or over the entire periphery of the conduit so that always the parts protruding in a direction to be effective for the erosion generate sufficient disturbance in the water flow for burying erosion. For example, in order to guide a downwardly directed swirl generated at the luff side and directed downwardly along the wall of the conduit, an embodiment was disclosed wherein the conduit had a radial longitudinal fin in an inclined position in the upper part e.g. at an angle of about 45.degree. to the horizontal direction. An even better guiding of said swirl was obtained by curving such a fin concavely downwardly. If the water flow was alternating as by the tide, there would normally be two such fins on the conduit.
Several advances in the use of protrusions or fins to promote self-burial of pipelines are disclosed in European Patent Specification No. EP 0466219 B1, entitled “Pipeline with Ground Anchors,” which is incorporated by reference herein in its entirety. That European patent disclosed a “seat” and “spoiler” structure in which the spoilers would be mounted to a seat that would “grab” across part of the outer surface of a pipe. The seat and spoiler would then be kept in place on the pipe by metal straps.
In practice, the spoilers or fins typically are aligned approximately over dead center, slightly spaced along the length of a section of pipe 110 such as is shown in FIGS. 1 and 3. The spoilers typically have a two-piece structure having a seat or saddle 112 and a fin 114, such as is shown in FIG. 2. The fins and seats typically are secured to the pipe 110 using metal straps.
A further advance was disclosed in European Patent Specification No. EP 0717223 B1, entitled “Assembly of pipelines intended to lie on a seabed and saddle for securing a second pipeline to a first pipeline,” which is incorporated by reference herein in its entirety. That patent disclosed a structure of the seat for protrusions from a pipeline in the form of a saddle for accommodating a second pipe.